1. Technical Field
The present invention relates to the field of reduction-oxidation (redox) flow batteries or redox flow cells and, in particular, to redox flow batteries that include an ion exchange membrane (IEM) that achieves an advantageous figure-of-merit (FOM) to improve Voltage and Coulombic Efficiency.
2. Discussion of Related Art
Reduction-oxidation (redox) flow batteries store electrical energy in a chemical form, and subsequently dispense the stored energy in an electrical form via a spontaneous reverse redox reaction. Aspects of redox flow batteries incorporating external liquid electrolytes have been described in, for example, U.S. Pat. No. 3,996,064 to Thaller, which is herein incorporated by reference in its entirety.
A redox flow battery is an electrochemical storage device in which an electrolyte containing one or more dissolved electro-active species flows through a reactor cell where chemical energy is converted to electrical energy. Conversely, the discharged electrolyte can be flowed through a reactor cell such that electrical energy is converted to chemical energy. Electrolyte is stored externally, for example in tanks, and flowed through a set of cells where the electrochemical reaction takes place. Externally stored electrolytes can be flowed through the battery system by pumping, gravity feed, or by any other method of moving fluid through the system. The reaction in a flow battery is reversible. The electrolyte, then, can be recharged without replacing the electroactive material. The energy capacity of a redox flow battery, therefore, is related to the total electrolyte volume, e.g., the size of the storage tank. The discharge time of a redox flow battery at full power also depends on electrolyte volume and often varies from several minutes to many days.
The minimal unit that performs the electrochemical energy conversion is generally called a “cell,” whether in the case of flow batteries, fuel cells, or secondary batteries. A device that integrates many such cells, coupled electrically in series or parallel, to get higher current or voltage or both, is generally called a “battery.” However, it is common to refer to any collection of coupled cells, including a single cell used on its own, as a battery. As such, a single cell can be referred to interchangeably as a “cell” or a “battery.”
Redox flow batteries can be utilized in many technologies that require the storage of electrical energy. For example, redox flow batteries can be utilized for storage of night-time electricity that is inexpensive to produce to subsequently provide electricity during peak demand when electricity is more expensive to produce or demand is beyond the capability of current production. Such batteries can also be utilized for storage of green energy, i.e., energy generated from renewable sources such as wind, solar, wave, or other non-conventional sources.
Many devices that operate on electricity are adversely affected by the sudden removal of their power supply. Flow redox batteries can be utilized as uninterruptible power supplies in place of more expensive backup generators. Efficient methods of power storage can be used to construct devices having a built-in backup that mitigates the effects of power cuts or sudden power failures. Power storage devices can also reduce the impact of a failure in a generating station.
Other situations where uninterruptible power supplies can be of importance include, but are not limited to, buildings where uninterrupted power is critical, such as hospitals. Such batteries can also be utilized for providing an uninterruptible power supply in developing countries, many of which do not have reliable electrical power sources, resulting in intermittent power availability. Another possible use for redox flow batteries is in electric vehicles. Electric vehicles can be rapidly “recharged” by replacing the electrolyte. The electrolyte can be recharged separately from the vehicle and reused.
There is, therefore, a need for improved performance and longevity of redox flow cells and the batteries formed from them.